Dark energy's long shadow

Einstein dismissed his theory about anti-gravity as one of his big mistakes. But, says Roger Highfield, he might have been right all along.

The universe is really repulsive. Albert Einstein first suggested something like this more than eight decades ago, but wrote it off as a monumental blunder. Theoreticians revived the idea in the 1980s, but it faltered due to lack of evidence. Today, there has been a big revival of the idea of a mysterious anti-gravity force that is tearing apart the universe, putting cosmology into a spin.

As theoreticians knit together the theories that describe the forces, energies and particles at work around us into an all-encompassing Theory of Everything, some could be forgiven for thinking that we are nearing the end of cosmology, where all that is left to do is tie up a few loose ends.

Think again. Over the past few years, this complacent view has been overturned by a deceptively simple answer to a deceptively simple question: if you draw a vast triangle across the cosmos, will its three angles add up to 180 degrees, as they do on a flat sheet of paper? Thanks to Boomerang and Maxima, balloon-borne telescopes, an Antarctic observatory called Dasi, and other experiments to measure tiny temperature variations, astronomers have deduced that the universe's geometry is indeed flat - it obeys what mathematicians call Euclidean geometry that pre-dates the Greek mathematician Euclid (c. 300 BC).

But for this to be so, cosmologists have come to a humbling conclusion: only 4 per cent of the universe consists of atoms and molecules. To flatten it out, the remaining 96 per cent consists of an invisible blend of additional repulsive and attractive forces. Astronomer Royal, Sir Martin Rees, says this is "an embarrassment . . . ordinary matter seems just an afterthought". About 23 per cent of the cosmos is "dark matter", an unexplained source of gravity. Candidates fall into a variety of categories, notably Machos (massive astronomical compact halo objects), which are underweight, failed stars. Some, such as cool white dwarfs and brown dwarfs, have been seen (but nowhere near enough). Other main contenders are called Wimps (weakly interacting massive particles), relic sub-atomic particles from the Big Bang of creation.

Even more disturbing is the deduction that about 73 per cent of the cosmos is dark energy, an antigravity force. Its identity is even more elusive, marking one of the greatest conundrums in contemporary science. According to Rees, while the mystery of dark matter is likely to be solved in the next five years, that of dark energy "is a surprise and a much deeper challenge".

Dark energy is related to an idea (the Cosmological Constant) introduced by Einstein in 1917, when he used its repulsive effects to balance the tug of all the matter in the universe to ensure that it was unchanging, the prevalent view of a static cosmos that is now known to be wrong. Even then, most astronomers doubted its reality, and Einstein came to call his constant the greatest blunder of his career.

Then, two decades ago, a theory called cosmic inflation proposed that the universe had a gigantic growth spurt just moments after the Big Bang. From the beginning, inflation's key prediction - a flat universe - was in trouble. Key experimental support for this - and a repulsive force akin to Einstein's reviled constant - had to wait for careful studies of distant exploding stars in 1998, which showed how the expansion of the universe is being accelerated by an antigravity effect. The implication was that over vast reaches of the universe, the influence of dark energy dominates over that of dark matter and ordinary matter.

A few weeks ago, an international team provided independent evidence of this repulsive effect - and of Einstein's constant - by using a telescope to map one-quarter of the entire sky, determining the positions and brightness of 100 million celestial objects. They revealed the imprint of dark energy by correlating clusters of galaxies on this map with one produced by the Wilkinson Microwave Anisotropy Probe, located a million miles from Earth, which studies the fading heat "echo" - microwave radiation - of the Big Bang of creation with unprecedented precision.

The team found "the shadow of dark energy" in the form of a slight boost in the energy of the microwave radiation from the Big Bang as it passes through huge clouds of galaxies, said Dr Bob Nichol of Carnegie Mellon University in Pittsburgh.

The combined sky maps revealed to the team how, as particles of light from the Big Bang's echo travel towards us, their passage can be influenced by dark energy - via its effects on "superclusters" of galaxies - through what is called the Integrated Sachs-Wolfe effect.

When the particles pass through a vast cloud of galaxies, their energy will first increase, as a rolling marble speeds up when it falls into a dip in the road. Later, as they leave the cloud, gravity will take away some energy, just as the marble climbs out of the dip, explains Dr Andrew Connolly, also of the Pittsburgh team.

If the universe was flat and matter dominated, these effects would cancel. But not if dark energy is tearing galaxies apart. Then the galaxy clouds become less dense rather than more dense as the particles pass through, allowing the dip to become more shallow and the particles to gain energy. This odd effect, where the shadow of dark energy is brighter than its surroundings, is what was actually observed. Nichol says that dark energy will not, as some articles have claimed, split the universe apart. The implications of a universe bathed in a weak repulsive force, are profound. Perhaps Einstein's theory of gravity (general relativity) is wrong and will have to be rejigged. Perhaps his theory is incomplete, not taking into account quantum mechanics. Or, perhaps, a new kind of theory is needed to explain this all-pervasive source of negative gravity.

Some even talk of a chameleon-like energy, "quintessence", that varies over time and space. The shadow of dark energy has made the universe a more perplexing place.